The actors playing Valjean and Javert are the only actors in Les Miserables that don't play any additional roles in the show.
I always chuckled at Kyle Scatliffe (Enjolras) because when he came out as a beggar in "At the End of the Day" they tried to hide him by having him cover most of his face with a blanket/shawl type thing, but he was so tall and it was so obviously him that it was almost comical.
Just taking Marius as an example - he's a prisoner in the opening, then also in the farm scene, then a beggar in, then one of the factory workers, then during Lovely Ladies / Fantine's Arrest he's one of the townspeople. Cosette and Eponine are also beggars, then factory workers, then whores during Lovely Ladies.